An X-ray CT apparatus applies X-rays from the vicinity of an object, and generates a distribution of X-ray absorption coefficients of the object as an image on the basis of projection data acquired at a plurality of projection angles. As an X-ray irradiation amount is increased, an image with reduced noise can be acquired, and thus image quality is improved. On the other hand, the influence of exposure to X-rays on a human body has been problematic in recent years, and thus a technique has been actively examined in which image quality necessary in a doctor's diagnosis is obtained in low dose scanning in which an X-ray irradiation amount is reduced.
Noise which influences an image obtained by the X-ray CT apparatus is roughly classified into photon noise caused by fluctuation of X-ray photons and system noise which is added in a data collecting system. The former changes according to an X-ray irradiation amount, but the latter exhibits an inherent noise level for each data collecting system. Thus, in a case where an amount of X-rays incident to a detector in low dose scanning is small, a ratio of the system noise occupying an output signal from the data collecting system increases.
PTL 1 discloses a technique in which, when measured data in which a ratio of the noise component is high due to a dose of X-rays incident to a detector being small is converted into projection data, the noise component is prevented from increasing through logarithmic conversion. In other words, in a case where a value of the measured data is equal to or greater than a predetermined value, the measured data is converted into projection data by using a logarithmic function in the same manner as in the related art. However, in a case where a dose of X-rays incident to the detector is small, and a value of the measured data is smaller than the predetermined value, the measured data is converted into projection data by using a function replacing the logarithmic function. Consequently, the noise component is suppressed from increasing and thus appearing as artifacts in an image when the measured data is converted into the projection data.
On the other hand, as techniques of reducing artifacts of an image caused by the photon noise and the system noise included in measured data, a technique of performing bias correction on the measured data or a technique of reconstructing an image through iterative reconstruction based on the measured data is also known.
The bias correction method is a method in which, as disclosed in NPL 1, a value of measured data in a focused element of a detector is corrected through iterative filtering processes while maintaining positivity of the focused element and an average value of the focused element and an adjacent element by referring to a value of measured data in the adjacent element of the focused element. The measured data having undergone the bias correction is converted into projection data by using a logarithmic function, and an image is reconstructed by using the projection data.
In the iterative reconstruction based on the measured data, as disclosed in NPL 2, the iterative reconstruction is performed on the measured data so that an image is reconstructed, without converting the measured data into projection data. Specifically, models of the photon noise and the system noise included in the measured data are generated, and an image is calculated by using an iterative solution on the basis of the models.